Nanoparticle suspension for inkjet printing magnetizable characters on a substrate

ABSTRACT

A fluid for ink jet printing characters on a substrate that become magnetized in the presence of a magnetic field, the fluid HAVING a suspension of nanoparticles dispersed in a solvent, wherein the fluid comprises a viscosity from 1 to 50 cps and a surface tension of 20-45 dynes/cm, further wherein each nanoparticle is sized between 10-180 nm and comprises M(III)2O3, M(II)O and M(II)M(III)2O4, wherein M(III) is a trivalent metal and M(II) is a divalent metal, or Fe2O3, MnO and M(II)O, wherein M is a divalent metal selected from the group consisting of Fe, Ni, Mn, Co, Cu, Pt, Au, Ag, Ba and a rare earth metal.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation in part of U.S. patent application Ser. No.13/410,283, now U.S. Pat. No. 9,390,846, filed Mar. 1, 2012, whichitself claims priority to U.S. provisional patent application Ser. No.61/447,874 filed Mar. 1, 2011; each of which is herein incorporated byreference in its entirety.

TECHNICAL FIELD

The invention relates to fluids for ink jet printing characters on asubstrate that become magnetized in the presence of a magnetic field andmore specifically to a suspension of nanoparticles suitable for highspeed and high resolution dot-on-demand inkjet printing, their use forprinting magnetizable characters and bio-molecularly code-labeledvariations thereof with directly or indirectly detectable IR, visiblelight and UV fluorescent properties.

BACKGROUND OF THE INVENTION

Inkjet printers are used to reproduce digital images on substrates bydelivering variably sized droplets of fluid, such as inks. These fluidsgenerally include a dye or pigment to form the desired color palettetogether with one or more solvents, whether aqueous or non-aqueous. Inaddition, they often include additional additives depending on desiredproperties. Among these include various surfactants, humectants,biocides, and dispersion aids.

Among the recent adaptations of inkjet printing is its use in theproduction of security documents, such as checks. To this end, fluidshave been supplemented with various additives to provide magnetizableproperties. Typically, these magnetizable fluids include iron oxide andare printed in special typefaces. The print is read by magnetizing thecharacters then passing them over a read head, which detects andinterprets particular waveforms. This process is referred to as MagneticInk Character Recognition (MICR). Among the benefits of MICR is that itpermits reliable reading of characters even when overprinted with stampsor other nonmagnetic inks. Thus, MICR provides an encouraging approachfor securing documents.

The widespread adoption of MICR has led to increased demand for improvedmagnetizable fluids; however, there remain significant challenges intheir development. Among these include challenges of increasing printspeed, resolution and shelf life. For instance, large magnetizableparticles tend to retain their imparted charge over longer lengths oftime and thus have a longer shelf life compared to smaller particles;however, solutions containing large particles tend to settle, slowprinting speeds and decrease resolution. Accordingly, there remains aneed to develop new magnetizable fluids for use with inkjet printingthat balance increased shelf life with increased print resolution andprint speed.

BRIEF SUMMARY OF THE INVENTION

The invention addresses the need to provide magnetic fluids for inkjetapplications, which have improved print speed and resolutioncharacteristics while providing long term shelf life. This isaccomplished by providing nanoparticle fluids for use with ink jetprinting on substrates for magnetic reading. The nanoparticle fluidincludes a suspension of nanoparticles including densely packedcomposites of metal oxides having metals of different oxidation states.Dense packing is achieved through nanosintering the metal oxides underhigh temperature and pressure, after which they are dispersed in asolvent and adjusted to achieve appropriate viscosity and surfacetension for inkjet printing. The nanoparticles are sized between 10 nmto 180 nm and suspended in a solvent to provide a viscosity from 1 to 50centipoise (cps) and a surface tension of 20-45 dynes/cm. Thenanoparticle fluid properties permit dispensing from an inkjet printerat a rate of at least 2.5 m/s and at a resolution of at least 600 dpi;however, a resolution of 2400 dpi has been achieved. The fluids may alsoinclude at least one biocide, surfactant or at least one substrateconditioner and optionally a co-solvent, polymeric binder, synergist,colorant, dye, pigment or hybrid thereof. The fluid can include UVcurable monomers or oligomers, a photo initiator, a biocide and/or asubstrate conditioner. Preferably, the fluids include a shelf life of atleast 12 months by resisting separation by a magnetic field orgravitational force. Resisting separation may be concluded if the fluidmaintains less than a 30% difference in density at both bottom and topregions of the fluid compared to the average density of the fluid.Preferably, the solvent includes at least 50% by volume an aqueoussolvent or a non-aqueous solvent.

The nanoparticles, which themselves also form a related aspect of theinvention, include at least two or more metal oxides, which may be thesame or different element, but have different valences or oxidationstates. Specifically, nanoparticles are formed from metal oxides havingmetals selected from two of the group consisting of a monovalent metal(Me+), a divalent metal (Me2+), a trivalent metal (Me3+), and aquadrivalent metal (Me4+). The metals can include rare earth metals,including lanthanides, scandium, and yttrium. In some embodiments themetal oxides include a FeIII oxide, a MnII oxide and an oxide of adivalent metal cation including a metal selected from the groupconsisting of Fe, Ni, Mn, Co, Cu, Pt, Au, Ag, Ba and a rare earth metal.At least one metal oxide may include FeIII or MnII. In a preferredembodiment, the nanoparticle includes M(III)₂O₃, M(II)O andM(II)M(III)₂O₄, where M(III) is a trivalent metal and M(II) is adivalent metal. In another preferred embodiment the nanoparticleincludes Fe₂O₃, MnO and M(II)O, wherein M is a divalent metal selectedfrom the group consisting of Fe, Ni, Mn, Co, Cu, Pt, Au, Ag, Ba and arare earth metal. In some embodiments the distance between any twodifferent neighboring metal oxides within the nanoparticle is 1 nm to 15nm.

In some embodiments the nanoparticles are further modified to include asecond detectable feature, which increases the security of printeddocuments. In particular, detectable labels can be attached to thenanoparticles through the use of binding surface moieties. In someembodiments, the nanoparticle has a surface moiety embodied as a nucleicacid molecule. In other embodiments, the nanoparticle has a surfacemoiety embodied as a polypeptide or a protein. In other embodiments, thesurface moiety includes phosphates, sulfates, silanes, phosphonic acids,or amines for bonding to a detectable label or to a labeled molecule.

The surface moieties themselves can be bound to both the nanoparticleand a detectable label; or they may be bound to the nanoparticle and toa complementary binding partner, which itself is bound to a detectablelabel.

In some embodiments, the fluid includes a mixture of differentnanoparticles, where a first set of nanoparticles has a blend ofdifferently labeled surface moieties and a second set of nanoparticleshas modifiable nanoparticles that can be modified with the same ordifferent labels.

In some embodiments, the fluid further includes C5-12 and C17-24multi-hydroxy multi-carbonic acids and a co-solvent selected from thegroup consisting of polyethylene glycol (PEG), glycerine, ethyleneglycol, a hydroxyl-(poly) ether, a hydroxyl-(poly) ketone, and ahydroxyl-(poly) aldehyde.

In another related aspect, the invention includes a substrate printedwith any of the fluids described. The fluid may be used for inkjetprinting, such as using a dot-on-demand process. In some embodiments thefluid is inkjet printed on a paper substrate, which can be porous ornon-porous, coated or non-coated. In some embodiments an applied perprint pass density of the printed substrate is at least 3.5 mg/inch. Insome embodiments the fluid may be printed on a paper substrate at aprint density of 5 μL/inch and at a black optical density of at least 90as measured using the TAPPI METHOD OD 1213.

In some embodiments the substrate is a document printed to include asecurity feature. In some embodiments the fluid is printed between atleast two layers of a multi-layer substrate, optionally whereinsurrounding layers are opaque and further conceal the fluid. In someembodiments, the fluid is covered with an opaque covering including amember selected from the group consisting of an ink, a dye, a pigment, acolored fluid, and an overcoat varnish. Detecting indicia printed usingthe fluid or reading data encoded by the printed indicia can beperformed by magnetization and subsequent magnetic scanning to detectthe magnetic waveform present and decode the waveform into itscorresponding data. Exemplary documents include certificates, currency,checks, tickets and stamps.

In further embodiments, the substrate is printed with a suspension oflabeled nanoparticles, where the label emits a detectable signal uponexcitation. In a preferred embodiment the label is excited by applying alight source at a particular wavelength or wavelength range to thelabel, which emits or fluoresces at a longer wavelength that is withinthe visible spectrum for ordinary optical detection by the user.Further, by incorporating mixtures of different labels, fluorescence ofa plurality of distinct colors over a wide spectrum has been achieved.In some embodiments, nanoparticles labeled with different fluorophoresare combined into a single suspension to alter the observed color afterexcitation. In other embodiments, a same nanoparticle is labeled with atleast two different fluorophores. In still another embodiment, thenanoparticle suspensions having different labels are stored in differentchambers within an inkjet cartridge and inkjet printed in closeproximity to yield an observable difference in color after excitation.

In a related aspect a machine readable mark used as a security featurefor an object is provided, the mark including the fluid printed on theobject and optionally covered by an opaque covering. Exemplary objectsinclude a variety of commercial goods such as electronic devices andtheir packaging, which includes an inkjet cartridge, a laser printercartridge, a rotating data carrier, a non-rotating data carrier, amemory chip, a compact disk a DVD jewel and other items that can beinkjet printed. Objects may also include a variety of household items,articles of clothing or other related articles that may be inkjetprinted. The machine readable mark may be printed in a configurationsuch as a 1-dimensional barcode, a 2-dimensional bar code, a PDF417barcode, a datamatrix barcode, in a E13B format and in a CMC7 format. Insome embodiments, the fluid is printed as a readable mark and inmultiple vertically stacked 2-dimensional dot arrays separated bythin-film substrates and where each array is selected from the groupconsisting of an encoded 2-dimensional pattern, a 1-dimensional barcode,a 2-dimensional bar code, a QR barcode, a PDF417 barcode, a datamatrixbarcode, a E13B format, a CMC7 format, and a magnetic ink characterrecognition (MICR) line on a check, ticket, tag and a label.

In another related aspect, use of the fluid in an offset printing methodis provided, which includes providing the fluid coupled to a printer,and printing indicia or a pattern on a paper, plastic or other substrateusing an offset printing process.

In another related aspect, a method of manufacturing the fluid isprovided, which includes: forming at least two different metal oxidesfrom at least two metals having different valences or oxidation states;precipitating the at least two different metal oxides together to form aprecipitate mixture; nanosintering the precipitate mixture to formnanoparticles; adding an agglomeration stabilizer and adjusting thesurface tension of the magnetic fluid suitable for use with inkjetprinting. The method may also include a consolidation step prior tonanosintering. Still further the methods may include adding one or moreco-solvents and surfactants or surface active agents after nanosinteringand optionally one or more colorants, dyes, pigments or hybrids thereof.In some embodiments the surfaces of the precipitated metal oxides arefunctionalized before nanosintering by adding one or more surfacemoieties, and after nanosintering the nanoparticle is labeled with oneor more detectable labels, such as fluorophores. In other embodiments,the nanoparticle is functionalized after the nanosintering step andlabeled with one or more detectable labels, such as fluorophores. Insome embodiments the consolidation step results in a carboxylic acidmoiety as the functionalize moiety for adding a detectable label.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic overview of the invention where 1.) Metal oxidesare precipitated alone in part, followed by 2.) combined precipitationand acid assisted consolidation and finally 3.) nanosintering of allprecipitated solutions to form a densely packed magnetic nanoparticlemetal composite.

FIG. 2 is a schematic showing a process of combining two individualmetal (i. Me(III), ii. Me(II)) hydroxide nanocrystalline precipitates(1-3 nm) to form larger precipitates of a third hydroxide solution ofmixed metals (7-20 nm). A.) Individual metals are precipitated andgrowth and nucleation is flash interrupted by a cryogenic fluid (liquidnitrogen (LN)). B.) Precipitated solutions are simultaneously ejecteddown a heated rod into a pressure reactor chamber under distinctconditions. C.) Anisotropic advanced crystal growth of mixed metalprecipitates occurs. The mixed metal precipitates are flash interruptedby LN, washed, and then exposed to an acid solution for acid assistedconsolidation.

FIG. 3 is a schematic showing a final process of mixing all threepredicated metal oxides (individual and mixed metals) with additionalreactant products to form a densely packed nanosintered magneticnanoparticle 10-180 nm. A.) The mixed metal acid consolidatedprecipitate solution (from FIG. 2) is washed and centrifuged and thenadded to a reaction pot with various reaction reagents, including moreconsolidation agents and individual metal precipitates. B.) The reactionsolution is ejected down a heated rod into a pressure reactor chamberunder distinct conditions. C.) Intimate particle proximity, undercrystal growth conditions causes nanoparticle formation. The particlesare flash interrupted by LN, washed, and then coated with surfacestabilizers or other surface active agents and are now ready forcombination with other ink components for a final ink formulation.

FIG. 4 provides a schematic depicting an exemplary final inkformulation. The fabricated nanoparticle fluid is combined with othercomponents and additives to make a final ink formula ready for storage.

DETAILED DESCRIPTION OF THE INVENTION

To assist the reader in understanding the terms used in the descriptionthe following provides a brief description of terms commonly referenced.

The tem “oxidation number” or “oxidation state” or “valence” as usedherein refers to oxidation state as known in the chemical arts, namely,the hypothetical charge that an atom would have if all bonds to the atomof were 100% ionic.

The term “metal element” or “element” as used herein refers to any metallisted on the periodic table. A “metal element” can have more than oneoxidation state.

The term “nanoparticle” or “particle” as used herein refers to adistinct complex formed by a plurality of metal oxides sintered togetherto form an object from 10 nm to 180 nm, where at least two metals differin oxidation state from one another.

The term “metal oxide” as used herein refers to a crystalline solid thatcontains a metal cation and an oxide anion. A “metal oxide” encompassesall metal oxides, including any monoxides, dioxides, trioxides,tetroxides, and pentoxides.

The term “rare earth metal” as used herein refers to the group offifteen lanthanides plus scandium and yttrium found on the periodictable.

The term “per print pass density” as used herein refers to the densityof fluid printed on a substrate in a single pass of an inkjet printerfrom a single nozzle.

As an introduction to the invention, a fluid suitable for printingcharacters for magnetic recognition having properties which permit highspeed and high resolution printing is provided. The fluid is useful inthe production of security features for substrates, such as documents orother objects where authentication is desired. The fluid is also usefulin the production of indicia used in tracking commercial products or ininventory control. It is an object of the invention to provide a fluidthat can be printed at high speeds and at high resolution. It is afurther object of the invention to provide a fluid having an extendedshelf life. These objects are accomplished through the development of afluid having a suspension of nanoparticles formed from densely packedmetal oxides, where the metals are provided in at least two differentoxidation states.

It has been surprisingly found that compared to conventional fluids formagnetic character recognition, the fluids herein have achieved inkjetdrop formation on a substrate at speeds of 10 m/sec and higher throughsmall diameter inkjet nozzles of 10 microns or less with shear rates ofabout 100,000 This permits surprisingly high speed jetting at highresolution of magnetizable particles. This is further surprising in viewof the extended shelf life of the fluid, which is greater than 12months. A variety of parameters have been tested experimentally. It hasalso been found that compared to current state of the art MICR inkformulations, implementations favor this new invention in terms ofuninterrupted duty cycle (e.g. Old vs. New: 10 times improvement),achievable print resolution (e.g. Old: 300<600 vs. New: up to 2400 dpi),drop eject frequency (e.g. Old: 12.5 KHz vs. New: 40 KHz), drop size(e.g. Old: >20 pl vs. New: >2 pl), permissible nozzle diameter (e.g.Old: >50 μM vs. New: >7 μM).

The fluid may be printed using a variety of inkjet printers and may beprinted on a variety of substrates depending on the needs of the user.As nonlimiting examples the fluids may be applied using a drop on demandsystem such as a variety of bubble jet printers, piezoelectric printersand other printing techniques that can use a printing fluid at 1 to 50cps and a surface tension of 20-45 dynes/cm. As such, the fluid may bestored in any delivery container suitable for use with the desiredprinter. The invention is particularly useful for printing securityfeatures associated with documents, certificates, currency and checks.It is also useful for security features associated with tickets andstamps. Commercially, vendors will use the fluids for security of theircommercial products or goods such as by printing the fluid directly onthe product or on a substrate associated with the product, such as aproduct box, sticker, label, tag or other substrate. Exemplary productsinclude electronic devices such as televisions, telephones, stereocomponents, computers, electronic accessories such as monitors,printers, inkjet cartridges, laser printer cartridges, rotating datacarriers, non-rotating data carriers, memory chips, compact disks, DVDjewels, and others. In addition, goods may include clothing or clothingaccessories, jewelry or accessories. In some instances, the magneticfluid is printed on products that require the purchaser to be over alegal age, such as on alcohol containers, tobacco products or otheradult products. Since the fluid itself is magnetizable, markings orindicia formed by the fluid discourage tampering such as intentionallyaltering indicia with fluids that are not magnetizable. Further, thefluid allows vendors or users to write or stamp over the printed indiciawithout concern of masking the data encoded by the fluid. For instance,a marking or indicia printed with a fluid can be covered by an opaquecovering, such as but not limited to an ink or a nonmagnetic substrate.

Preferably, the fluid includes a shelf life of at least 12 months byresisting separation from a magnetic field or gravitational force. Insome embodiments the fluid has a shelf life of over 18 months. In someembodiments the fluid has a shelf life of over 24 months. In someembodiments the fluid has a shelf life of three years or more. Shelflife can be determined by calculating a change in density to assesswhether the nanoparticles have settled from solution. This can beaccomplished by measuring the density of the fluid at top and bottomportions of a stored fluid at any given time point, subtracting thedensity of the top from the density of the bottom, dividing the resultby the average density of the fluid as a whole and multiplying theamount by 100%. As guidance, an exemplary threshold for determiningsuitable shelf life is a difference in density between top and bottomthat is within 30% of the average fluid density; however, a magneticfluid is more preferred if the value is between 20%-25% and is stillmore preferred if the value is less than 20%, and is most preferred ifthe value is less than 10%.

While the fluid can be printed in any desired configuration,configurations of particular interest may include a 1-dimensionalbarcode, a 2-dimensional bar code, a PDF417 barcode, a datamatrixbarcode, a E13B format and a CMC7 format. In other embodiments the fluidis printed as a readable mark and in multiple vertically stacked2-dimensional dot arrays separated by thin-film substrates and whereeach array is selected from the group consisting of an encoded2-dimensional pattern, a 1-dimensional barcode, a 2-dimensional barcode, a QR barcode, a PDF417 barcode, a datamatrix barcode, a E13Bformat, a CMC7 format, and a magnetic ink character recognition (MICR)line on a check, ticket, tag or label.

In some embodiments the fluid is printed directly on the top of thesubstrate but other embodiments, the fluid is printed between layers ofa substrate. The fluid may also be combined with other securityfeatures. For example, when printed together with other security basedfluids, such as ultraviolet detectable fluids or printing techniques,such as nonlinear or random printing of data strings, security of asubstrate or document can be enhanced. As an example, the fluid can beused in the production of a secured check, where the magnetic propertiesare positioned in multiple locations and in a nonlinear format and whereadditional data is printed in a UV excitable fluid.

I. Nanoparticle Fluids

The fluid itself includes a suspension of nanoparticles, which includesmetal oxides densely packed by nanosintering under elevated temperatureand pressure, then dispersed in a solvent and adjusted to appropriateviscosity and surface tension for printing. After printing, the printedindicia becomes magnetized in the presence of a magnetic field. Themetal oxides within the nanoparticle include at least two or more metalshaving different valences or oxidation states, each of which is selectedfrom a monovalent metal (Me+), a divalent metal (Me2+), a trivalentmetal (Me3+), and a quadrivalent metal (Me4+). Rare earth metals mayalso be used. In some embodiments the metal is a same metal elementhaving a different oxidation state; however, in others at least twometals are both different metal elements and have different oxidationstates. In some embodiments three metals having two or three differentoxidation states are used, optionally each being a different metalelement. In still further embodiments, four metals having two, three, orfour different oxidation states are used, optionally each being adifferent metal element.

The nanoparticle fluid has a viscosity and surface tension that permitsdispensing from an inkjet printer at a rate of 2.5 m/s and at aresolution of at least 600 dpi. More preferably the inks are printed ata resolution of 1200 dpi to about 2400 dpi. This is accomplished throughthe small but dense nanoparticle configuration. In preferredembodiments, the viscosity is 1 to 50 centipoise (cps), the surfacetension is 20 to 45 dynes/cm and the nanoparticle is sized from 10 to180 nm.

The metal oxides provide the printed indicia with its magneticproperties. Exemplary metal oxides that can be used include a FeIIIoxide, a MnII oxide and an oxide of a divalent metal cation that itselfincludes or is derived from a metal selected from the group consistingof Fe, Ni, Mn, Co, Cu, Pt, Au, Ag, Ba and a rare earth metal.Nanosintering results in the joining of the at least two metals havingdifferent oxidation states to one another to form nanoparticles suitablefor use as in the nanoparticle fluid. Metal oxides are preferablyprovided in equal or about equal amounts for forming the nanoparticlethereby resulting in an approximately equal ratio of metal oxides withinthe nanoparticles; however, the skilled artisan will appreciate thattheir distribution within the nanoparticle may vary. In otherembodiments the ratio between metal oxides varies within thenanoparticles. Additional guidance is provided by the following generalformula: Me1(alpha1)a1Me2(alpha2)a2 . . . Men(alphan)an-ON whereSUM[(alphai)*ai]=2N, with alphai is element of {1, 2, 3, 4}, ai iselement of Q>0 and Mei is element of the group of metals described aboveand O is Oxygen and N is an integer >0. Variations can be achieved bynanosintering different ratios of metal oxide populations. Typicallynanosintering results in a segment of about 1 nm to about 15 nm betweenneighboring metals with different oxidation states; however, inpreferred embodiments the average segment between neighboring metalshaving different oxidation states is from about 1 nm to about 7 nm. Bydensely packing the particles, increased loads for magnetization may beadded during each printing pass along a substrate or document within agiven volume. The result is an applied per print pass density of atleast 3.5 mg/inch². When testing on a paper substrate the nanoparticlefluid demonstrated a consistent print density of 5 μL/inch and a blackoptical density of at least 90 as measured using the TAPPI METHOD OD1213 and a densitometer. A density according to the TAPPI METHOD OD 1213is the optical-negative logarithm to base 10 of transmittance fortransparent material or the reflectance for an opaque material and hasthe equation Optical Density=log 10 1/R, where R is reflectance. Thesurface tension of the nanoparticle fluid is generally about 20-45 dynesand the viscosity is generally between about 1 cps to 50 cps for inkjetprinting; however, if printed using a different approach, such as an inkpad and press, these values can differ.

The formed nanoparticles may be stored or suspended in a variety ofsolvents depending on the desired use. For instance, the nanoparticlescan be suspended in a solvent including at least 50% by volume of anaqueous solvent or a non-aqueous solvent. The term “solvent” within thecontext of the invention refers to aqueous or non-aqueous and organic orinorganic solvents. Examples of solvents that may be used with theinvention include water, isopropanol, tetrahydrofuran (THF), acetone,hexane, petroleum, epoxy and the like. The solvent may be chosen in partdepending on the desired substrate as known in the inkjet arts.

When using the nanoparticle fluids in inkjet processes considerationmust be given to the fluid viscosity. A viscosity that is too high tendsto the clog the jets or tends to bead on the surface causing an unevenprinting. A viscosity that is too low tends to be “runny” when dispensedand reduces the control over the high resolution positioning of colorsor images. Accordingly, when using drop-on-demand type printers such asbubble jet printers and piezoelectric printers, the preferred viscosityis less than about 50 centipoise (cps). More preferably the viscosity isbetween 1-50 cps. The viscosity can be adjusted by thickeners,dispersants other approaches known in the inkjet arts. For example,thickeners tend to increase the viscosity thereby thickening the fluid.Dispersants however promote the distribution and separation of fine orextremely fine solid particles. Accordingly, dispersants may assist inuniformly distributing the magnetic particles throughout the fluidthereby preventing the fluid from settling. Settling can be determinedby measuring the density of the magnetic fluid and determining whetheror to what extent the density differs from bottom to top in a givenmagnetic fluid sample. Dispersants are typically provided between about1% and 10%. Dispersants are generally formed from a surfactant andsolvent. Generally the solvent carries the surfactant into the solutionand the surfactant reduces the surface tension and preventsagglomeration, such as in response to Van der Waals forces and magneticforces. Thus, a nanoparticle fluid having a higher capacity to impart amagnetic force may require more surfactant than a conventional inkjetfluid. Surfactants themselves are well known in the inkjet arts. Whileamounts can vary, surfactants are generally provided in an amount fromabout 0.1% to about 8%. Greater amounts of surfactants tend to reducesurface tension and thus increase wetting. The particular amount ofsurfactant may vary depending on the particular nanoparticles or thesubstrate used for printing.

In addition, a variety of additives may be provided, including but notlimited to one or more biocides, humectants or drying control additives,synergists, substrate conditioners or wetting additives, colorants orsurface appearance additives, pigments, other metallic or nonmetallicnanoparticles, dyes, polymer additives, anti-settling additives,dispersants, foam control, adhesion promotion additives, rheologycontrol additives and the like as known in the inkjet arts. In someembodiments ultra violet curable monomers or oligomers and initiatorsare provided in the magnetic fluid, which upon exposure to a UV lightsource forms a solid polymer.

Biocides and fungicides are chemical substances or compounds capable ofkilling living organisms. As such, their inclusion may prevent growth orattachment of living organisms to the fluid thereby increasing itslifespan. Biocides and fungicides may be synthetic or natural and maythemselves have short or long half-lives. A variety of biocides andfungicides are known in the inkjet arts and thus their inclusion andrelative amounts may be determined by the ordinarily skilled artisan.While amounts can vary, generally biocides and fungicides form about0.01% to about 1% of the fluid. In some embodiments biocides alsoprovide surfactant or humectant properties.

Humectants are compounds that reduce evaporation and are often providedas a co-solvent together with a dispersant. Accordingly, humectantsassist in retaining a consistent concentration of particles throughoutthe shelf life of the fluid. An exemplary humectant is ethylene glycol.Examples of preferred co-solvents include ethylene glycol, polyethyleneglycol (PEG), glycerine, hydroxyl-(poly) ether, hydroxyl-(poly) ketone,a hydroxyl-(poly) aldehyde and others known in the inkjet arts. As anonlimiting example, humectants are usually provided between 0% and lessthan 1%; however, greater amounts such as about 5%, 10%, 15% and 20% arealso encompassed by the invention.

The nanoparticle fluid may also include UV curable monomers oroligomers, a photo initiator, and a substrate conditioner depending onthe intended end use. Accordingly, UV curable monomers or oligomers mayundergo a polymerization reaction to polymerize the fluid compositionalong the substrate, which decrease drying time. A variety of polymersare known in the art such as acrylates, including methacrylates,hydroxyethyl methacrylate, and others. Further, hydrophobic polymers arepolymers that do not absorb an appreciable amount of an aqueoussolution; whereas hydrophilic polymers do tend to absorb an appreciableamount of aqueous solution. UV curing is generally accomplished byapplying a UV source to a photo initiator, which begins thepolymerization reaction. In an exemplary method of use, the magneticfluid including UV curable monomers or oligomers are applied to asubstrate, such as using an inkjet printer. The nanoparticle fluid iscured by applying a UV source to initiate radical formation and thuspolymerization. UV light sources can be coupled directly to an ink jetprinter or may be provided separately. In another approach thenanoparticle fluid incorporates oxidative polymerization to assist indrying of the fluid. In oxidative polymerization oxygen from the airadds to double bonds within the fluid to form hydroperoxides. Like UVinduced polymerization, the resulting free radicals attack othermolecules and form new larger free radicals. This causes polymerizationof the fluid. Whether a UV curable approach is used or oxidativepolymerization or cationic polymerization, adding monomers or oligomersthat can undergo polymerization to the magnetic fluid can reduce dryingtime of the magnetic fluid.

Colorants may also be added to change or enhance the coloring of thenanoparticle fluid. One skilled in the art will appreciate that avariety of approaches are used to impart color, which include the use ofcolorants, dyes, pigments, and hybrids thereof. Accordingly, the term“colorant” as used herein can also refer to dyes, pigments, and hybridsthereof as used in the inkjet arts. Dyes are generally applied in anaqueous fluid and typically have an affinity for the substrate; whereas,pigments are generally insoluble and do not have affinity for thesubstrate. While dyes may be used, generally pigments are preferred.Each can be used as desired by the user and as consistent with theparticular formulation. Colorants are generally provided in an amount ofabout 1% to 15% although they may be used in higher amounts such as 20%,25%. 30% or more.

A sample of a typical water-based inkjet fluid that can be used with thenanoparticles is shown in Table 1 below:

TABLE 1 Component Composition Purpose Water 50-90%  Solvent, carrierfluid Colorants-Dyes and Pigments 0-10% Add vibrancy Co-solvents 5-50%Humectants, enhance ink formation on paper Surfactants 0-5%  Improvewetting and flow through nozzle Polymeric binders 0-10% Increasedurability and appearance of print Other additives: biocides 0-1% Prevent bacteria growth

Printing indicia with a nanoparticle suspension lacking added colorantprovides a colorless and transparent print, which provides addedsecurity for authenticating documents by not immediately revealing theposition of the magnetizable indicia. However, further variations havebeen made to the nanoparticle to provide still further securityfeatures. For example, in some embodiments the nanoparticle is labeledwith a molecule, such as a fluorophore that emits a wavelength in thevisible spectrum in response to exposure to a shorter excitationwavelength. In this embodiment, printed indicia provides both magnetismand fluorescence for document authentication. In further embodiments, acollection of nanoparticles are labeled with different fluorophores anddeposited together on a substrate so they emit blended colors uponexcitation. By blending colors or using print with different fluorescentcolors, the secured document can have a color profile forauthentication. Such nanoparticle suspensions can be adapted toconventional inkjet cartridges akin to the conventional colors red,yellow and cyan, by providing red, yellow and cyan fluorescence and/orother fluorescent colors.

Most fluorophores are organic small molecules of 200-1000 Da, with some,such as Green fluorescent protein (GFP) and phycobiliporteins (PE, APC)being about 27 kDa and 240 kDa respectively. Depending on the printmethod, nonlimiting examples of fluorescent polypeptides or proteinsthat may be usable with the nanoparticle suspension include bluefluorescent protein (BFP), cyan fluorescent protein (CFP), greenfluorescent protein (GFP), enhanced green fluorescent protein (EGFP),red fluorescent protein (RFP or R-PE or PE-Cy5), yellow fluorescentprotein (YFP), and others known in the art to which these belong.

In other embodiments, the fluorophore is a compound but not a protein ora polypeptide. Non-protein based fluorophores typically have severalcombined aromatic rings or are themselves cyclic. As nonlimitingexamples, the fluorophore may be selected from xanthene derivatives(e.g. fluorescein, rhodamine), cyanine derivatives (e.g. cyanine,indocabocyanine), squaranine derivatives, naphthalene derivatives,coumarin, derivatives, oxadiazole derivatives, anthracene derivatives,pyrene derivatives, oxazine derivatives, acridine derivatives,arylmethine derivatives and tetrapyrrole derivatives.

Fluorescent molecules can be conjugated to the nanoparticle byfunctionalizing the surface of the nanoparticle then reacting thefluorophore with the functionalize surface. For instance, a surfacefunctionalized with amines can be made to bond to a fluorophore via anamide bond. In another approach, a surface functionalized withcarboxylic acid moieties is reacted with an N-terminus of a polypeptide.Ester chemistries may also be used to bind labels to nanoparticles. Inanother approach a surface functionalized with single strandedoligonucleotides can be reacted with a fluorophore conjugated to acomplementary nucleic acid strand. In still another approach thenanoparticle is biotinylated then reacted with a fluorophore conjugatedto an avidin (e.g. avidin, streptavidin, ultra avidin) or vice versa. Instill another embodiment a surface protein is immobilized on the surfaceof the nanoparticle, and a labeled antibody fragment against the surfaceprotein is added to label the nanoparticle. To this end, thenanoparticle may have a surface moiety that is a nucleic acid molecule,a polypeptide, a protein or a molecule having a complementary bindingpartner. The nanoparticle may itself have a surface moiety with adetectable label.

In some embodiments, the fluid includes a mixture of differentnanoparticles, where a first set of nanoparticles has a blend ofdifferently labeled surface moieties or fluorescent labels and a secondset of nanoparticles has modifiable nanoparticles that can be modifiedwith different labels, such as by adding desired labels. To this end,mixtures of different labels can be applied to a substrate having aunique fluorescent profile.

Functionalizing the surface for subsequent label attachment can beperformed before or after nanoparticle formation. Functional groups suchas nucleic acids, carboxylic acids, phosphates, sulfates, silanes,phosphonic acids, amines, etc. are suitable as moieties for addingdetectable labels. Functionalization methods themselves may includemicroemulsions, redox transmetalization, sol gel methods, and so on tocreate core with shell. Au, Ag, Pt, Pd, SiO₂, and C are common shellmaterials for magnetic nanoparticles to introduce additional surfacefunctionality.

In a preferred approach, functionalization includes exposing singlestranded nucleic acids, whether DNA, RNA or cDNA to the nanoparticle forabsorption. Alternatively, partially double stranded nucleic acidmolecules can be absorbed, then one stranded melted from the other toproduce a free single strand. Melting temperature of nucleic acidstrands depends generally on the length of the nucleic acid molecule(longer oligos have higher melting temperatures than shorter oligos) andguanine—cytosine (GC) content compared to the adenine-thymine (AT)content (GC having a higher melting temperature because GC has threenucleotide bonds whereas AT has two). Still further, mismatching basepairs can lower the melting temperature.

Once absorbed single stranded nucleic acid molecules can accept acomplementary nucleic acid strand that itself is labeled for binding. Insuch an approach, labels that emit different colors in response toexposure to a frequency spectrum can be combined to produce signatureexcitation profiles. That is, by functionalizing the surface of thenanoparticle with mixtures of different oligonucleotide sequences,different complementary oligonucleotides can be selectively added to thenanoparticle to selectively label the nanoparticle with differentlabeling molecules. Joining different labels can be arranged accordingto annealing temperature of the complementary nucleic acid strands.

II. Uses for Nanoparticle Fluids

While the nanoparticle fluid can be used for printing on nearly anysubstrate, the invention is particularly useful for printing securityfeatures associated with documents, certificates, currency, checks,commercial products and other substrates with security features. In someembodiments, the nanoparticle fluid is used as a means for magnetic inkcharacter recognition, such for printing MICR lines on checks. In suchembodiments the nanoparticle fluid is printed on a check such that whenthe check moves across a magnetic head the magnetic head detects theflux of magnetic signal and when combined with the elapsed read time,obtains a characteristic curve in accordance with the change in magneticflux. While the MICR approach in general is known in the art; thenanoparticle fluid of the present invention has a higher capacity toretain magnetic forces yet provides smaller magnetizable particles,which facilitates improved resolution, a higher signal to area ratio,higher speed printing and higher speed of recognition. Still further,while conventional MICR lines are printed along the bottom of the checkand in linear format, the nanoparticle fluid may be printed in otherlocations and in other configurations. For example the nanoparticlesuspension can be printed to enhance security by printing in variousfont sizes, in a non-linear configuration, a curved configuration, anangled configuration or in randomized places on the document. Inaddition, conventional MICR lines and associated printing inks havealways been colored. In contrast, the nanoparticle suspension and itsprinted indicia is transparent and colorless unless a colorant is added.That is, the nanoparticle suspension cannot be seen by the naked eyeunless provided in an embodiment with an added visible colorant. To thisend, indicia printed with the nanoparticle suspension is typicallyinvisible without further aid thereby improving its security.

In some embodiments the fluid is printed directly on a commercialproduct such as directly on a device itself or its packaging. Thus, usesof the nanoparticle fluid expand beyond banking uses and extend intoproduct security, inventory control and the like through theincorporation of the nanoparticle fluid together with a magnetic reader.

While the nanoparticle fluid may be used alone, in other embodiments thenanoparticle fluid is used together with a second fluid, such asnomagnetizable ink, which in some instances is applied over thenanoparticle fluid to hide or visually mask magnetic features or toimpart color to indicia printed in nanoparticle fluid to enhance itsvisibility or identification. Accordingly, in a related aspect of theinvention, a substrate having the nanoparticle fluid printed thereon isprovided, which optionally also includes a second fluid such as acolored non-magnetizable fluid also printed thereon. In otherembodiments the nanoparticle fluid is provided together with additionalsecurity fluids, such as UV excitable fluids that are exposed by theapplication of a UV source, IR excitable fluids that are exposed by theapplication of a IR source, colored fluids in non-linear formats, suchas random non-linear printing of security data and the like.

The nanoparticle fluids can be printed using an inkjet printer. Oneskilled in the art will appreciate that the nanoparticle fluid can beprovided in a suitable housing, such as a print cartridge as dictated bythe printer. Similarly, one skilled in the art will appreciate that thenanoparticle fluid can be printed in a variety of formats orconfigurations through the use of suitable print software. For example,nanoparticle fluid can be used to print a mark such as a 1-dimensionalbarcode, a 2-dimensional bar code, a PDF417 barcode, a datamatrixbarcode, indicia in E13B format, CMC7 format or the like. In someembodiments the fluid is printed as a readable mark and in multiplevertically stacked 2-dimensional dot arrays separated by thin-filmsubstrates and where each array is selected from the group consisting ofan encoded 2-dimensional pattern, a 1-dimensional barcode, a2-dimensional bar code, a QR barcode, a PDF417 barcode, a datamatrixbarcode, a E13B format, a CMC7 format, and a magnetic ink characterrecognition (MICR) line on a check, ticket, tag or label.

In some embodiments, the user generates an image using suitable imagedrawing software and prints the image on a substrate using thenanoparticle suspension or fluid. Such technologies generally involvegenerating one or more bitmaps. For example, in some approaches a bitmapis generated for printing magnetizable characters using the nanoparticlefluid and a second bitmap is generated for printing a non-magnetizablecharacters. Examples of non-magnetizable fluids include a variety offluids generally referred to as inks that include colorants, pigments,dyes or a mixture thereof to provide visible indicia. Although theskilled artisan will appreciate that the nanoparticle suspension itselfmay also include one or more colorant, pigment, dye or mixture thereof.In any event, one skilled in the art will recognize that printing thenanoparticle fluid on a substrate results in a magnetically readablemark, which can be detected by a suitable magnetic reader. Generallysuch magnetic readers are coupled to a computer to interpret themagnetically encoded data such as converting magnetic waveforms intocorresponding data.

While the nanoparticle fluid can be printed directly on a substrate oron top of a layered substrate, in some embodiments the nanoparticlefluid is layered between at least two layers of a substrate since themagnetic properties in some embodiments are detectable through thesubstrate. In further embodiments, the substrate includes three or morelayers with the nanoparticle fluid printed between two or more of thelayers. While the substrate can be transparent or semi-transparent oftenthe substrate will be opaque. In some instances the substrate is amulti-layer laminate. In such instances, inkjet printing using thenanoparticle fluid can be performed during the lamination process.Accordingly, markings may be printed between layers, which associate theparticular laminate with its corresponding characteristics,identification, authentication or other usable data as needed by theuser. While useful for securing documents, the nanoparticle fluid or afluid containing the densely packed nanoparticles may also be adaptedfor biomedical applications. For example, densely packed nanoparticlesthat are biocompatible and non-toxic may be used in vivo as contrastagents in magnetic resonance imaging (MRI), labeling of tissues orcells, such as cancerous tissues, magnetic control of transport oftherapeutics, thermotherapy and the like. The nanoparticle fluid mayalso be used ex vivo or in vitro to detect or measure interactionsbetween molecules, such as through coupling the densely packednanoparticles to compounds, polypeptides and the like followed by assayssuch as binding assays known in the chemical and biochemical arts.

Nanoparticle fluids suitable for use in MM include the densely packednanoparticles formed from metals provided in two different oxidationstates that are suspended in a suitable solvent or media. The skilledartisan will appreciate that depending on the intended use or route ofadministration, the formulation may deviate from the formulations usedin ink jet printing. For example, MM contrasting agents would notgenerally incorporate colorants, dyes or inkjet solvents and may beprovided at a biological pH, use biocompatible solvents and deliveryvehicles known in the Mill field. When coupling the densely packednanoparticle to compounds, such as to antibodies for targeting aparticular protein or cell type, such as a cancer cell in a heattreatment, the particle can be adapted with reactive groups thatfacilitate protein or nucleic acid conjugation.

III. Methods of Manufacturing Nanoparticle Fluids

In a related aspect of the invention, a method of manufacturing ananoparticle fluid is provided. The method includes i. providing atleast two metals having different valences or oxidation states as threesamples, wherein a first sample includes the first metal, a secondsample includes the second metal and a third sample includes both thefirst and second metal, such as an consolidated alloy; ii. forming metalhydroxides from the metals in each of the samples; iii. combining thethree samples to form a mixture of different metal hydroxides; iv.nanosintering the mixture of metal hydroxides to form densely packedcomposite nanoparticles; and v. coating nanoparticles with low molecularweight hydroxyl carbonic acids to act as agglomeration stabilizers.

In preferred embodiments, metals used in the manufacturing of thenanoparticle fluid have different valences or different oxidationstates. Most preferably, the two or three metals are selected from twoof the group consisting of a monovalent metal (Me+), a divalent metal(Me2+), a trivalent metal (Me3+), and a quadrivalent metal (Me4+). Themetals can be rare earth metals. Rare earth metals are generally in a 3+oxidation state; however, europium also has a valence of 2+ and ceriumalso has a valence of 4+. Nonetheless, at least one and up to fourmetals having two different valences or oxidation states are selected,which may be from different metal elements having different oxidationstates or same metal elements having different oxidation states. Metalsare generally added to an aqueous solvent in the form of a salt(chlorides, sulfates, nitrates, etc.), to begin the transformation intocross-crystallized nanoparticles. Possible metal combinations andgeneral stoichiometric formulations for cross-crystallized magneticnanoparticles are as follows:

$\begin{matrix}{{{Fe}({III})},{{Fe}({II})},\left. {{Mn}({II})}\rightarrow{{{Fe}({III})}_{2}{{Fe}({II})}_{0.5}{Mn}_{0.5}O_{4}} \right.} & \left. {a.} \right) \\{{{Fe}({III})},{{Fe}({II})},\left. {{Co}({II})}\rightarrow{{{Fe}({III})}_{2}{{Fe}({II})}_{0.3}{Co}_{0.7}O_{4}} \right.} & \left. {b.} \right) \\{{{Ni}({II})},{{Fe}({III})},\left. {{Co}({II})}\rightarrow{{Fe}_{2}{Ni}_{0.3}{Co}_{0.7}O_{4}} \right.} & \left. {c.} \right) \\{{{{Ni}({II})},{{Fe}({III})},\left. {{Mn}({II})}\rightarrow{{Fe}_{2}{Ni}_{0.3}{Mn}_{0.7}O_{4}} \right.}{{General}\text{:}}} & \left. {d.} \right) \\{\left( {{{Me}({III})}_{2}{{Me}({II})}_{1 - b}{{Me}({II})}_{b}} \right){O_{4}\left( {{{Me}(I)}{{Me}({II})}_{1 - b}({MeIII})_{1 - {({2\text{/}3b})}}({MeIV})_{2}} \right)}O_{5}} & \left. {e.} \right)\end{matrix}$

Initially, metals can be precipitated into metal hydroxide precipitatesby hydroxide displacement of a corresponding acidic inorganic anion ofthe metal salt, which is later followed by rapid dehydration to form ametal (Me) oxide. In the preferred embodiments at least three metalhydroxide solutions are precipitated including at least two individualmetal hydroxides and one combined metal hydroxide solution (i.e. Fe₂O₃,MnO, and Fe₂MnO₄). The first precipitation of the at least twoindividual metal hydroxides results in 1-3 nm metal crystallites whichare combined and precipitated again to form the third hydroxide solutionwith 7-15 nm particles. In preferred embodiments precipitation isinterrupted by flash freezing with a cryogenic fluid once theprecipitation reaction results in particles of about 7-15 nm.Determining or confirming particle size can be conducted using a varietyof methods, such as various filtration techniques to exclude particlesthat are too large, milling to ensure the particles are not too large orother approaches known in the inkjet arts. Preferably milling isavoided. A series of centrifugation steps can also be used to separatedifferent sized particles. Characterization techniques such astransmission electron microscopy (TEM) or high resolution scanningelectron microscopy (SEM) can also provide images to determine size.Zetasizers or other particle size analyzers can also be utilized forachieving particle size distributions, etc. After the precipitationinterruption at the desired size, the three solution samples (twoindividual metal hydroxide solutions (1-3 nm crystallites) and acollective metal hydroxide solution (7-15 nm particles) can be combinedby acid consolidation to form a compiled mixture of precipitated metalhydroxides for downstream additional precipitation reactions andeventually nanosintering. In the preferred embodiments, the optionalstep of acid consolidation is based on acid surface active agents whichassist in providing a short-range force on the particles for bringingthem closer to one another. Ultimately, the acid consolidation aids indensification of the particles.

A final mixture of precipitated metal oxides is then nanosintered toform an ultimate nanoparticle (10-180 nm) of both metals. In thepreferred embodiments, nanosintering is initiated by ejecting at leastthree precipitated fluids simultaneously towards a heating rod heated to40-90 degrees C. The fluid is collected in a chamber where thenanoparticles undergo a low pressure (<100 MPa) sintering stage withtemperatures between 80-120 degree C. It should be noted that higherpressures (up to 1 GPa) can also be used achieving the desirednanosintered atomic structure. Ultrasound frequencies around 60 kW/m3together with an induced magnetic field (created by permanent magnetsnear the reaction chamber) assist in densification during thenanosintering process. Preferably, nanosintering results in highly densenanoparticles such that magenetizable properties of printed indicia maybe prolonged thereby increasing its shelf life while also increasing themagnetic capacity of the particles. In preferred embodiments thedistance between neighboring metals that have different oxidationstates, which is also referred to as the intra particle segment is about1 nm to about 15 nm and preferably an overall particle size is about10-180 nm. Achieving unexpectedly dense particles of such small size isachieved by the precipitation and sintering approach. Specifically,sintering can be enhanced by altering the pH of the solution in a rangebetween 10-14, increasing the temperature of the solution to over 40°C., more preferably to about 120° C., applying ultrasound at a frequencyabove 20 KHz (greater than 60 KW/m³), and reducing the spatialdistribution of the precipitate by applying a magnetic field of 0.1-1 T,preferably 0.5 T to the particles during the sintering process. In somecases, the magnetic field is realized by placing two permanent magnetsof 40 kGauss at the bottom and 10 kGauss at the top of the reactionchamber. The magnetic field affects the nanoparticles and influences thetransport through the media during the nanosintering process. Any suchmethod to polarize the electron clouds and create electron shells in themedia could be utilized during nanosintering to influence thenanoparticles. Furthermore, nanosintering is a result of anisotropicadvanced crystal growth under confined conditions.

FIG. 1 reveals a schematic overview of the invention where 1.) Metalprecursors produce metal crystallites 1-3 nm in diameter 2.) Combinedprecipitation and acid consolidation of the two metals creates 7-15 nmparticles consisting of a combination of two metals and 3.)nanosintering occurs when additional metal crystallite seeds are addedto the consolidated metal precipitated mixtures under discretetemperatures, pressure, ultrasound frequency, and magnetic fields toproduce nanoparticle metal composites 10-180 nm in size.

Once nanosintered, preferably the nanoparticles have a size or diameterof about 10-180 nm but more preferably 10-60 nm. While the methodsthemselves produce a high purity of such nanoparticles, filtration ofthe densely packed nanoparticles can confirm and optionally select forthe desired size. In preferred embodiments, in the process ofconsolidating nanoparticles prior to nanosintering of the nanoparticlesas well as afterward, it is beneficial to stabilize the particles byapplying active surface agents (surfactants, ligands, small polarmolecules, polymers), preferably acid surface agents such ascarbon-based molecules rich in carbonic acids and/or hydroxyl moieties.Among these include C5-12 and C17-C24 multi-hydroxy multi carbonicacids. Preferably, for aqueous citrate ion ligands (1-100 mg/ml) provideelectrostatic repulsion for stabilization and for non-aqueous fattyacids such as oleic acid (0.1-100 mM) provide steric repulsion forstabilization. In some cases the combination of electrostatic and stericstabilization is found, for example in the case of some polymers ordendrimers. In other cases the addition of multiple layers of surfaceagents may be required. In general, surface active reagents can bechemically anchored or physically adsorbed on the densely packednanoparticles to form a repulsive force to balance the van der Waalsattractive forces acting on the nanoparticles, thus stabilization in asuspension is achieved. Functional groups including but not limited tonucleic acids, carboxylic acids, phosphates, sulfates, silanes,phosphonic acids, amines, etc. are suitable as surface activefunctionalities. Those skilled in the art of nanoparticle synthesis willappreciate that precious metals and other coatings can be deposited ondensely packed nanoparticles through reactions in microemulsions, redoxtransmetalization, sol gel methods, and so on to create core—shellmagnetic nanoparticles if desired post sintering. Au, Ag, Pt, Pd, SiO₂,and C are common shell materials for magnetic nanoparticles to introduceadditional surface functionality.

Functionalizing the surface of the nanoparticle with a surface moietycan improve the security of the documents printed with the nanoparticlesuspension. By functionalizing the surface, the nanoparticle can belabeled with molecules detectable by means other than magnetization,which provides at least two forms of identification for authentication.In some embodiments, the carboxylic acid group is joined to an aminoterminus of a polypeptide or a protein, which itself is detectable. Inanother embodiment the polypeptide or protein is conjugated to thefunctionalized surface of the nanoparticle using a different conjugationchemistry. In some embodiments, amide linkages bind the polypeptide tothe nanoparticle.

Nonlimiting examples of polypeptides or proteins that may be usable withthe nanoparticle include blue fluorescent protein (BFP), cyanfluorescent protein (CFP), green fluorescent protein (GFP), enhancedgreen fluorescent protein (EGFP), red fluorescent protein (RFP or R-PEor PE-Cy5), yellow fluorescent protein (YFP), and others known in theart to which these belong.

In another embodiment, the label is a fluorescent molecule that is not aprotein or amino acid. These typically have several combined aromaticrings or are cyclic. As nonlimiting examples, the molecules may beselected from xanthene derivatives (e.g. fluorescein, rhodamine),cyanine derivatives (e.g. cyanine, indocabocyanine), squaraninederivatives, naphthalene derivatives, coumarin, derivatives, oxadiazolederivatives, anthracene derivatives, pyrene derivatives, oxazinederivatives, acridine derivatives, arylmethine derivatives andtetrapyrrole derivatives.

An alternative and/or additional approach to functionalization includesexposing single stranded nucleic acids, whether DNA, RNA or cDNA to thenanoparticle for absorption. This can be performed at room temperaturefor about 30 minutes. Alternatively, partially double stranded nucleicacid molecules can be absorbed, then one stranded melted from the otherto produce a free single strand for binding. Melting temperature ofnucleic acid strands depends generally on the length of the nucleic acidmolecule (longer oligos have higher melting temperatures than shorteroligos) and guanine—cytosine (GC) content compared to theadenine-thymine (AT) content (GC having a higher melting temperaturebecause GC has three nucleotide bonds whereas AT has two). Stillfurther, mismatching base pairs can lower the melting temperature.

Once absorbed nucleic acid strands can accept a complementary nucleicacid strand that itself is labeled for binding. For instance, it is wellknown that guanine (G) bonds cytosine (C) and adenine (A) bonds tothymine (T). In such an approach, labels that emit different colors inresponse to exposure to a frequency spectrum can be combined to producesignature excitation profiles. That is, by functionalizing the surfaceof the nanoparticle with mixtures of different oligonucleotidesequences, different complementary oligonucleotides with differentlabels can be selectively added to the nanoparticle to selectively labelthe nanoparticle with different label molecules. It has also been foundthat the addition of a co-solvent can improve the properties of themagnetic fluid when using it in the application of inkjet printing.Preferably, the co-solvent includes polyethylene glycol (PEG).glycerine, ethylene glycol, a hydroxyl-(poly) ether, a hydroxyl-(poly)ketone, a hydroxyl-(poly) aldehyde or the like.

Adapting the nanoparticle fluid for a variety of different purposes,print nozzles or for variations that improve shelf life may also includethe addition of UV curable monomers or oligomers, a photoinitiator, abiocide, a bonding agent, an anti-corrosion agent, a substrateconditioner, buffer reagents to control pH or the like as known in theinkjet arts. Additives may also be provided to enhance or alter thecolor of the magnetic fluid. Among these include a variety of colorants,pigments, dyes or a mixture thereof.

Example 1 Formation of Densely Packed Nanoparticles for Use in aMagnetizable Fluid

In this example, i.) metal oxides of Me(III) and Me(II) are precipitatedalone in part, followed by ii.) combined precipitation. The combinedprecipitates then undergo iii.) acid assisted consolidation followed byiv.) nanosintering. Afterwards, colorants and additives can be providedto the nanosintered oxide structures as desired to create a finalnanoparticle fluid for desired applications.

Materials and Methods. First, nano-crystallites of metals are preparedwith a wet reduction method. A suspension of Me(III) and Me(II) saltsolutions (i.e. Me(chlorides, sulfates, nitrates, etc.)) are eachindependently precipitated in an inert atmosphere using a base such asNaOH, increasing the pH of the system between 10-14, for 1-5 hours atelevated temperatures of 40-120 degrees C. to allow nucleation of verysmall nanocrystals that are 1-3 nm in diameter. Nucleation ofnanocrystals is followed by flash interruption of growth by a cryogenicliquid, such as liquid nitrogen (LN), to form Me oxide particles of 1 nmto about 3 nm (FIG. 2, panel (a)) A third suspension of both Me(III) andMe(II) can also be precipitated under the same conditions.

The solutions are combined by ejecting them simultaneously toward a rodheated to 40-90 degree C. and precipitation continues as the fluids arecollected in an autoclave with a pressure <100 MPa and temperaturebetween 80-120 degree C. (FIG. 2, panel (b)). Ultrasonic frequenciesof >20 KHz and a magnetic field magnetic field (H-field (Gauss)) createdby placing two permanent magnets of 5 kG at the top and 40 kG at thebottom of the reaction chamber assist in anisoptropic advanced growth ofthe particles (FIG. 2, panel (c)). The particles then undergo washingand acidic assisted consolidation in aqueous C5-10 and non-aqueousC16-24, about 1-100 mg/ml or (0-1M) concentration in appropriate medium,for 1-5 hrs at 40-120 degrees C. Acid consolidation is repeated multipletimes in aqueous C5-10 and non aqueous C16-24. In preferred cases,during this step at least two carboxyl groups are dissociated and form acarboxylate complex with the Me atoms on the surface. The additivemolecules, such as maleic acid or fumaric acid have a chemical moietywhich is able to bind to the nanoparticle surface, control the particlegrowth and eventually yield to a preferred size. The growth is flashinterrupted when the particles reach about 10-180 nm.

The consolidated metal nanoparticle precipitate solution is washed andfurther precipitation is allowed to occur with newly added reactants,which consists of metal precipitate solutions and acid consolidationsolutions, by ejecting a mixture of various precipitates down a heatedrod in the same way as described previously, as shown in FIG. 3 panels(a-b). Next, particles are fused by nanosintering in an autoclave (FIG.3 panel (c)) under the same conditions as described previously in water,as an example of an aqueous solvent or particles can be nanosintered inepoxy/oil as a non-aqueous solvent, etc.—Intimate particle proximity,with aid of acid consolidation, under crystal growth conditions causesnanoparticle formation. The process is flash interrupted by liquidnitrogen when the nanoparticles reach a diameter of 10-180 nm, Thenanoparticles are washed and centrifuged and a final surface stabilizeror coating is applied to the nanoparticle surfaces. Finally, themagnetic nanoparticle fluid is combined with other ink components andadditives and prepared for final storage as shown in FIG. 4. The naturalpigmentation is a result of the cross-crystallization and colors of thefinal Me nanoparticle composite.

What is claimed is:
 1. A fluid for ink jet printing characters on asubstrate that become magnetized in the presence of a magnetic field,the fluid comprising a suspension of nanoparticles dispersed in asolvent, wherein the fluid comprises a viscosity from 1 to 50 cps and asurface tension of 20-45 dynes/cm, further wherein each nanoparticle issized between 10-180 nm and comprises: a) M(III)₂O₃, M(II)O andM(II)M(III)₂O₄, wherein M(III) is a trivalent metal and M(II) is adivalent metal, or b) Fe₂O₃, MnO and M(II)O, wherein M is a divalentmetal selected from the group consisting of Fe, Ni, Mn, Co, Cu, Pt, Au,Ag, Ba and a rare earth metal.
 2. The fluid according to claim 1,wherein the nanoparticles further comprise a surface moiety comprising anucleic acid molecule.
 3. The fluid according to claim 1, wherein thenanoparticles further comprise a surface moiety comprising a polypeptideor a protein.
 4. The fluid according to claim 1, wherein thenanoparticles further comprise a surface moiety with a detectable label.5. The fluid according to claim 1, wherein the fluid comprises a mixtureof different nanoparticles, wherein a first set of nanoparticlescomprises a blend of differently labeled surface moieties and a secondset of nanoparticles comprises a modifiable nanoparticles that can bemodified with different labels.
 6. The fluid according to claim 1,wherein the fluid further comprises C5-12 and C17-24 multi-hydroxymulti-carbonic acids and a co-solvent selected from the group consistingof polyethylene glycol (PEG), glycerine, ethylene glycol, ahydroxyl-(poly) ether, a hydroxyl-(poly) ketone, and a hydroxyl-(poly)aldehyde.
 7. The fluid according to claim 1, wherein the distancebetween any two different neighboring metal oxides with the nanoparticleis 1 nm to 15 nm.
 8. The fluid according to claim 1, wherein the solventcomprises at least 50% by volume an aqueous solvent or a non-aqueoussolvent, the fluid further comprising at least one biocide or at leastone substrate conditioner, a bonding agent and optionally a colorant,pigment, dye or pigment-dye hybrid.
 9. The fluid according to claim 1,further comprising UV curable monomers or oligomers, a photoinitiator, abiocide and a substrate conditioner.
 10. The fluid according to claim 1,wherein the fluid comprises a shelf life of at least 12 months byresisting separation by a magnetic field or gravitational force, whereinresisting separation comprises maintaining less than a 30% difference indensity at both bottom and top regions of the fluid compared to theaverage density of the fluid.
 11. A substrate printed with the fluidaccording to claim
 1. 12. The substrate according to claim 11,comprising print density of at least 5 μL/inch² and a black opticaldensity of at least 90 as measured using the TAPPI METHOD OD
 1213. 13.The substrate according to claim 11, wherein the substrate is a layereddocument and the fluid provides a security feature, wherein the fluid isdeposited between at least two of layers of the document, wherein the atleast two layers are opaque.
 14. The substrate according to claim 11,wherein the document is selected from the group consisting of acertificate, currency, a check, a ticket and a stamp.
 15. The substrateaccording to claim 11, wherein the fluid is covered with an opaquecovering including a member selected from the group consisting of anink, a dye, a pigment, a colored fluid, and an overcoat varnish.
 16. Thesubstrate according to claim 11, wherein the fluid is printed as areadable mark and in a configuration selected from the group consistingof an encoded 2-dimensional dot pattern, a 1-dimensional barcode, a2-dimensional bar code, a QR barcode, a PDF417 barcode, a datamatrixbarcode, a E13B format, a CMC7 format, and a magnetic ink characterrecognition (MICR) line on a check.
 17. The substrate according to claim11, wherein the fluid is printed as a readable mark and in multiplevertically stacked 2-dimensional dot arrays separated by thin-filmsubstrates and where each array is selected from the group consisting ofan encoded 2-dimensional pattern, a 1-dimensional barcode, a2-dimensional bar code, a QR barcode, a PDF417 barcode, a datamatrixbarcode, a E13B format, a CMC7 format, and a magnetic ink characterrecognition (MICR) line on a check, ticket, tag or label.